命定悖论或命运悖论（Predestination paradox，命运注定的悖论），又称因果循环（Causal loop或Causality loop，与佛教的业德去返无关），是科幻作品中与时间旅行有关的悖论。

这悖论大致与祖父悖论相同：

命定悖论是其中一个去解释「为何历史事实不被时间旅行中所作出的改变而受影响」的方法，就是：

诺维科夫自洽性原则提出，只有事件属前后一致的因果循环才能出现，矛盾的则不能。

A causal loop, in the context of time travel or retrocausality, is a sequence of events (actions, information, objects, people) in which an event is among the causes of another event, which in turn is among the causes of the first-mentioned event. Such causally-looped events then exist in spacetime, but their origin cannot be determined. A theorized example of a causality loop is given of a billiard ball striking its past self: the billiard ball moves in a path towards a time machine, and the future self of the billiard ball emerges from the time machine before its past self enters it, giving its past self a glancing blow, altering the past ball's path and causing it to enter the time machine at an angle that would cause its future self to strike its past self the very glancing blow that altered its path.